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Abstract: An extensive field trial was set up in eight forest stands to examine the influence of soil texture (two stands on sand, four on loam to silt loam, two on clay), machine mass (light, heavy) and traffic intensity (one and five skidding cycles) (i.e. pass back and forth on the skid trail) on soil compaction after mechanized harvesting. Dry bulk density (BD), penetration resistance (PR), micro-topography and soil carbon dioxide (CO2) concentration were applied as response variables for soil compaction. Significant effects on BD were nearly absent (<7% increase) and occurred occasionally for PR (60–70% increase, up to 150% on clay soils). Especially for loam to silt loam and clay soils, this was in contrast with the expectation. The negligible compaction degrees for loam to silt loam are attributed to high initial compaction levels that prevented further compaction, as was found by general linear modelling (GLM) for both BD and PR. For clay soils the small compaction degrees can be explained by the high water contents that result in plastic deformation instead of strong compaction degrees, as was confirmed by the micro-topographical measurements. GLM also revealed a significant impact of machine mass (BD) and soil water content (BD, PR) on the compaction degree. Soil texture, traffic intensity and position in relation to the wheel tracks generally turned out to have an insignificant influence. With regard to clear interactions the influence of traffic intensity depends on the position in relation to the wheel tracks and the machine that was used (PR). In contrast with BD and PR, soil CO2 concentration, measured in a forest stand on sand, showed significant increases within and between wheel tracks, even after one skidding cycle. Although soil compaction degrees were small to negligible, machine passes apparently had a strong negative impact on pore continuity. CO2 concentration seems to be a more sensitive and thus better indicator for soil compaction. [Copyright &y& Elsevier]

Abstract: Impact of skidding traffic on bulk density and penetration resistance of two sandy forest soils was examined in Putte (the Netherlands). Different levels of compaction were applied by varying the number of skidding cycles: one pass harvester (H), one pass harvester and forwarder (H+F), and multiple passes of both machines (Max). Bulk density and penetration resistance were measured on the undisturbed surface (UD), between the wheel tracks (BT) and within the tracks (WT). For WT, treatment H induced a clear increase of both soil properties in the upper 30cm of the soil profile compared to the UD. The continuation of the passes to Max only resulted in a limited rise in bulk density. However, penetration resistance was significantly higher in Max in comparison with H. BT values were situated between UD and WT. Here, on site 1 both properties decreased with increasing number of passes, while on site 2, we observed the opposite, which was probably associated with the soil water status at time of harvesting. Measurements taken within tracks where logging residues were piled up to 40cm revealed that a brash mat could reduce the compaction level to a considerable extent. The relationship between bulk density and penetration resistance appeared to be non-linear, with bulk density becoming insensitive to penetration resistance changes at higher penetration resistance values. On these sandy soils, we recorded significant increases of bulk density and penetration resistance, but rarely exceeding growth limits for optimal root elongation. However, as amongst other things, sandy soils are expected to recover very slowly and certain soil processes may already be influenced at lower compaction levels, designated skid trails should also on this soil type be used to minimize the influence on the ecosystem. [Copyright &y& Elsevier]